Staurozoa

Staurozoa

Overview

Description

Staurozoa is a class of the phylum Cnidaria containing roughly 50 species commonly known as stalked jellyfishes or stauromedusae (the Greek roots stauro‐ and ‐medusa for cross‐shaped and the gorgon with snakes for hair, respectively). Staurozoans are smallish animals (1‐4 cm) that live in marine environments, usually attached to seaweeds, rocks, or gravel. The group has a largely anti‐tropical distribution, with the majority of species being found in boreal or polar, near‐shore, and shallow waters. Nevertheless, some species inhabit the deep sea, including at least one that is associated with mid‐ocean ridge habitats adjacent to hydrothermal vents. In addition, a small number of species have been observed in warmer tropical and subtropical water environments of the Atlantic, Indian, and Pacific Ocean basins. The vast majority of species are known from the northern hemisphere, but this may reflect study effort rather than a real geographic pattern in diversity. The 50 or so species of Staurozoa are presently classified in one order, six families, and 14 genera.

Description

Morphology

All staurozoans possess a stalk (also known as a peduncle) for adhering to substrates. The rest of the staurozoan body is called the calyx. The adult body shapes of different staurozoan species can be trumpet‐like to worm‐like, but most are perhaps best described as having a goblet form with fourparted symmetry. Looking down into the calyx, one can see a raised, central mouth with four distinctive corners. Four funnel‐like depressions, or infundibula, are visible 45 degrees askew of the corners of the mouth. These funnels mark the beginning of four longitudinal muscles that run down into the stalk. Distributed around the edge of the calyx are eight sets of tentacles ending in little knobs containing concentrations of nematocysts (the characteristic stinging cells of all cnidarians). These sets of tentacles are sometimes on extensions of the calyx known as arms and are often arranged in pairs. In some species, however, arms and tentacle sets are evenly distributed, creating an impression of eight‐parted symmetry. The eight sets of tentacles are termed secondary to differentiate them from eight primary tentacles that arise earlier in the development of juveniles, one each between the positions of the eight sets of secondary tentacles. Primary tentacles are often lost by adulthood, but many species retain them, and in one group they are further modified into hollow structures known as anchors because of their adhesive qualities. Many species bear distinctive patterns of white nematocyst spots upon the calyx. Some staurozoans are quite colorful, but their colors often mimic the algae upon which they live, rendering them rather cryptic. Staurozoan gonads are contained within the gut and often organized in sac‐like ovaries that are visible from the outside of the partly translucent calyx.

Ecology and Distribution

Ecology

Staurozoans are known to eat a variety of small prey items (worms, snails, amphipods, copepods, etc.), but the primary diet appears to be small crustaceans. Different species exhibit varying levels of activity, with some waving around quite vigorously and others remaining rather rigid. Prey items can be taken either from the surrounding water or off of nearby substrate when they come into contact with the secondary tentacles. Even though damaged specimens are fairly common in the field, not much is known about what species prey upon staurozoans. Both fish and nudibranch seaslugs have been observed to eat different staurozoans, but much remains to be learned in this regard. Similarly, little is known about the ecological health of different species. Historical literature describes them as abundant and easily encountered in shallow waters of north Atlantic localities where they are no longer found. Because of the proximity of large cities, it is tempting to conclude that habitat degradation due to human activities is responsible for these apparent declines, but greater knowledge of staurozoan ecology is needed in order to assess this hypothesis.

Life Cycle

The life cycles of different staurozoan species are still not known in great detail, but a general picture is emerging. Most appear to have an annual life cycle, with populations peaking during spring, summer or fall. Some species appear to be gregarious, occurring in large groups with patchy distributions, whereas others have only been encountered in isolation. After rapid growth, adult males and females shed their sperm and eggs, respectively, into the sea over an extended period (in contrast to many marine species that spawn largely on a single day). Eggs are negatively buoyant and therefore sink and settle not far from where they were released. After fertilization, the zygote develops into an elongated planula larval form. Unlike the planula of most cnidarians, those of staurozoans are not ciliated and move about in an inchworm‐like fashion. A recently discovered, minute (less than 0.5 mm) stage capable of asexually producing numerous creeping larvae is present in at least one species. Creeping larvae settle and grow into a juvenile polyp with eight primary tentacles. In turn, the juvenile polyp undergoes a metamorphosis, often involving the resorption of the primary tentacles, the growth of secondary tentacles, gonads, and distinctive features in the gut called gastic cirri. These finger‐like projections are loaded with nematocysts and aid in digestion.

Evolution and Systematics

Evolution

As the common name stalked jellyfish reflects, staurozoans have long been thought to be allied with the true jellyfish (of class Scyphozoa), with which they share a number of features. Like scyphozoan jellyfish, and their close relatives the box jellyfish (class Cubozoa), staurozoans possess gastric cirri. However, unlike scyphozoans and cubozoans, stalked jellyfishes do not have an adult swimming medusa stage. They spend the entirety of their lives attached to bottom substrates, and in this regard resemble cnidarians of class Anthozoa (anemones, corals, and kin). From the earliest inquiry into staurozoans, this combination of characteristics, some anemone‐like and others jellyfish‐like, intrigued marine biologists. In the late 1800's, scientists armed with the new paradigm of Darwin's natural selection used evolutionary arguments to conclude that staurozoans were most likely descended from within the true jellyfish and that they were in a sense "degenerate" forms. However, recent studies using fossils, morphology, and genetic data converge have falsified this view and instead suggest that Staurozoa arose earlier than the groups with swimming jellyfish. This is why the group is now considered a distinct class within the phylum Cnidaria.